The prior art in this area has suffered from the lack of an energy efficient process for producing ultrafine ground calcium carbonate. Further, a great deal of difficulty has been experienced in producing a suitable product capable of being spray dried and producing an easily dispersed pellet or bead.
The prior art teaches the employment of a dispersing chemical to enable calcium carbonate to be ground to a finer particle size than is possible in a non-dispersed system. Much of the prior art teaches the use of dispersants such as glassy phosphate which is slowly reactive with calcium carbonate. The reaction tends to produce calcium phosphate and/or calcium biphosphate which causes reagglomeration of the fine calcite particles into hard-to-disperse coarse particles which are unsuitable for coating paper.
The prior art also teaches processes depending on a coagulating chemical such as aluminum sulfate or calcium chloride for dewatering. The aluminum sulfate is slowly reactive with calcium carbonate thus forming an aluminum hydrate pigment which may promote reagglomeration of the fine particles. The residual sulfate ions increase the viscosity of the finished product and cause an increased demand for dispersing chemicals. The calcium chloride used for coagulation purposes does not produce the state of flocculation provided by alum and it releases residual chloride ions to the calcium carbonate which can result in higher viscosities and higher dispersant demands.
The additional problem related to the prior art is the tendency to ignore otherwise suitable deposits of calcium carbonate as the raw material for an ultrafine ground calcium carbonate product because of the lack of effective leaching techniques applicable to calcium carbonate as found in its various natural states.
Methods are known in the prior art for treatment of calcium carbonate-type materials to improve their characteristics. Thus, U.S. Pat. No. 4,165,840 to Lewis et al describes a method for processing calcitic ores to produce a finely divided calcium carbonate of high brightness characteristics by initially coarse-milling to produce a product where no more than 5% by weight of the particulate is +325 mesh, and no more than 35% by weight of the particles are less than 2 microns in size. This coarse-milled product is then subjected as a slurry to froth flotation with the purified underflow then being dewatered to at least 60% solids by weight and then wet-milled as in a sandmill to yield an output product wherein at least 80% of the particulate is a size of less than 2 microns.
U.S. Pat. No. 3,661,610 teaches a similar process to produce a mixed pigment comprising clay and calcium carbonate by a method comprising forming an aqueous suspension of the clay containing a first dispersing agent which is a water soluble phosphate, and a second dispersing agent which is a water-soluble salt of an organic macromolecular anion, forming an aqueous suspension of the calcium carbonate pigment and then blending the two solutions.
British Patent No. 1,285,891 discloses methods for improving the brightness of alkaline earth metal carbonates such as calcium carbonate by treating the carbonates at 20.degree.-100.degree.C. with either a halogen, an oxygen containing acid of a halogen, or a salt of such an acid, ozone or hydrogen peroxide. Thus, this patent discloses methods for brightening calcium carbonate by contact with a brightening agent such as ozone.